Owing to their particular properties, gels are used in many fields, in particular the food section, the cosmetics field or the pharmaceutical field. A gel is composed of at least two components, one of which, which is very highly predominant, corresponds to a liquid solvent and the other of which is a component that can be described as solid. The two components are continuous throughout the entire medium. The “solid” phase constitutes a network which traps the “liquid” phase corresponding to the solvent, and prevents it from flowing. The medium as a whole behaves like a soft and elastic solid that is easy to deform.
Gels can be classified according to the type of links which form the network. Thus, two major gelling mechanisms can be distinguished, which result in “physical” gels or in “chemical” gels. Starting from a solution or from a dispersion in the liquid state, the formation of the gel is the result of the formation of a continuous solid network. This transformation is called solution/gel transition.
A physical gel is a supramolecular assembly constituted of molecules linked to one another by low-energy bonds (Van der Waals, hydrogen bonds, polar bonds, etc.). The stability of this assembly is associated with a precise range of physicochemical conditions (pH, concentration of molecules, temperature, solvent quality, ionic strength, etc.). Outside this range, the mixture is liquid. The sol/gel transition is therefore reversible for physical gels. Thus, a modification of the parameters of the medium can lead to the destruction of the structure and induce a gel/sol transition. Compositions for obtaining “physical” gels are well known from the prior art. Biogels are obtained essentially from macromolecules or polymers of natural origin: proteins or polysaccharides.
Gels described as “chemical” are also known in the prior art. A chemical gel corresponds to a supramolecular assembly, the molecules of which are associated by high-energy bonds (covalent bonds). The stability of this assembly is therefore very high. These chemical gels exhibit improved stability, the only means of performing a gel/solution transition consisting in destroying the covalent bonds of the network. For this reason, the sol/gel transition of chemical gels is said to be irreversible.
One family of chemical gels corresponds to enzymatically catalyzed gels. This gelling mode is especially observed in the major biological processes. Blood clotting, healing, skin formation and extracellular matrix assembly are biological processes where the change of soluble proteins into the gel state is essential. In vivo, a limited number of enzymes, for example lysyloxidases and transglutaminases, catalyze these reactions. In vitro, the most used is transglutaminase, which creates covalent bridges between the side chains of the lysine and glutamine residues of proteins.
Tgases thus catalyze the polymerization of proteins responsible for the formation of biological gelled networks. This family of proteins is ubiquitous and it is found both in prokaryotes and eukaryotes. Tgases make it possible to obtain gels from many proteins in the food industry, and in particular for manufacturing surimi or hardening many meat derivatives (ham, reconstituted food, etc.). By way of example of polymerizable proteins, mention may be made of gelatin, fibrin, gliadin, myosin, globulin (7S and 11 S), actin, myoglobin, whey proteins, in particular caseins and lactoglobulin, soy proteins, wheat proteins, and in particular glutenin, egg white and egg yolk, and in particular ovalbumin.
One of the protein gels most widely used is the gelatin gel. Gelatin is obtained from collagen, which is a structural protein. Collagen is a molecule that is organized into a triple helix. These triple helices can associate to form fibrils, which can associate to form fibers. The collagen triple helix is unstable at body temperature. Gelatin is obtained by denaturing collagen. The tissues containing collagen thus undergo an acid or alkaline treatment, which results in the denaturation of the collagen triple helix. The possibility of making fibers is then completely lost. An acid treatment results in formation of gelatin type A and an alkaline treatment results in a gelatin type B. The gelatin solution is therefore composed of isolated collagen chains. Since there are many uses for gelatin, it is sometimes necessary to create gelatin gels under conditions where physical gels do not exist (high temperatures, extreme pH or particular ionic strength). In order to form the network necessary for the gel, the gelatin chains are then bridged by covalent bonds, and in particular by the action of Tgases. The gels thus obtained are chemical gels. Greater control of the mechanical properties of the various chemical gels therefore constitutes essential stakes for extending their potentiality.
Analysis of the living world has revealed the existence of extremely dynamic systems. In living tissues, the cells are interacting with a structure called the extracellular matrix (ECM), which is rich in proteins and can be likened to a gel at the macroscopic level. This structure is mainly located under epithelial cells and around connective tissues. The cells can synthesize various extracellular matrix components, such as collagen, which confers its rigidity on the ECM, or fibronectin, which is involved in cell adhesion mechanisms. In parallel, the cell also produces proteases which generate extracellular matrix degradation. The cell is therefore simultaneously involved in the construction and degradation of the extracellular matrix. The structure of the extracellular matrix is not, therefore, an irreversible and static structure, but corresponds to a dynamic equilibrium resulting from the balance between the activities of construction and of degradation of the proteins synthesized by the cell.
Similarly, the clots formed according to the blood clotting mechanism also constitute dynamic systems. Thus, via a cascade of enzyme reactions, a clot is formed from soluble proteins which become organized in an insoluble network. This clot will then be eliminated during another enzyme reaction.
In these dynamic equilibria, the protein networks associate so as to become insoluble and form gels, which can be likened to solution/gel transitions. At the same time, the protein networks are also destroyed by the action of proteases, it being possible for this type of transition to this time be likened to gel/solution transitions. Successive transitions are thus sometimes witnessed, as in clotting, where the clot is first of all formed, and then degraded. Solution/gel transition in these biological processes is most commonly associated with the transglutaminase family mentioned above. The opposite transition, namely gel/solution, is associated with the antagonist activity of enzymes of the proteolytic type.
One of the most widely studied families is that of the matrix metalloproteinases (MMPs). They form a family of zinc-dependent endopeptidases which degrade most extracellular matrix proteins. However, a large number of different proteases exist. By way of example of families of proteases, mention may be made of serine proteases, such as trypsin or matriptase, cysteine and aspartate proteases, such as cathepsins B and L and cathepsins D and G, metalloproteases and the ADAM family. A large number of the enzymes orchestrating this type of reaction, such as transglutaminases or alternatively metalloproteases, have been characterized by biochemists and enzymologists.
Gels having the capacity for solution/gel and gel/solution transition are described in document WO 2006/056700. These gels comprise an aqueous phase, a polymer, and enzymes capable of degrading the polymer and of polymerizing monomers in order to form said polymer. In document WO 2006/056700, the “monomers” can be biological macromolecules or polymers. In addition, in this document, the term “polymer” applies to a “network of polymers”.
The prior art gels exhibit programmed gelling and resolubilizing kinetics. Moreover, the prior art gels exhibit controlled physical characteristics, for example their viscoelasticity. Furthermore, the physical characteristics of the solid network and of the aqueous phase forming the prior art gels are indissociably and simultaneously modified. These drawbacks limit the field of application of these gels owing to their physical characteristics. It is, for example, impossible, according to the prior art, to modify the solid network without modifying the aqueous phase, or to modify the aqueous phase without modifying the solid network of gel.
There exists therefore a real need for novel biomaterials, in particular in the form of gels, of which the physical properties of just one of the two phases constituting the gel can be modified in a controlled manner. There also exists a need for novel gelled biomaterials capable of incorporating molecules, in particular active molecules (for example, cosmetic and/or pharmaceutical molecules), and of releasing these molecules in a controlled manner by modifying, in a controlled manner, the physical properties of just one of the two phases constituting the gelled biomaterial.